In cellular networks, e.g., as specified by 3GPP (3rd Generation Partnership Project), it is known to use broadcast or multicast transmission modes for efficiently conveying data, e.g., multimedia content such as mobile TV, to a plurality of users. For example, such multicast or broadcast transmission modes typically allow simultaneous reception of the data by a large number of users while avoiding excessive usage of radio resources because multiple users may receive the data from the same radio resources. An example of such broadcast or multicast transmission mode eMBMS (evolved Multimedia Broadcast Multicast Service) as specified in 3GPP TS 36.440 V11.2.0 (2013 March) for the LTE (Long Term Evolution) radio technology.
In eMBMS, a BM-SC (Broadcast Multicast Service Centre) provides multicast data to a plurality of LTE base stations, referred to as eNB (evolved Node B) and instructs the eNBs when to send the multicast data over the radio interface. In this way, the multicast data can be transmitted synchronously by the eNBs on a single frequency. The synchronization is achieved by utilizing a synchronization protocol as specified in 3GPP TS 25.446 V11.0.0 (2012 September). According to this synchronization protocol, the multicast data is transmitted in Sync PDUs (Packet Data Units) which include a relative timestamp indicating the starting time of sending the multicast data by the eNBs. The relative timestamp is calculated by the BM-SC as the time when the Sync PDU leaves the BM-SC plus an offset. The offset has the purpose of ensuring that the Sync PDUs reach the different eNBs before the intended starting time, irrespective of an inevitable delay incurred during transmission of the Sync PDU to each individual eNB.
The performance of eMBMS is sensitive to the setting of the offset. For example, if the offset is too short, there is a risk that the Sync PDU will not reach certain eNBs in time, i.e., sufficiently early before the intended starting time of sending the multicast data. Such eNBs may then not participate in the transmission of the multicast data in the Sync PDU and will typically drop the Sync PDU, preventing the users served by these eNBs from receiving the multicast data. On the other hand, if the offset is too long the eNBs typically need to buffer more Sync PDUs, which increases usage of memory resources at the eNBs. Still further, the offset also causes a delay in the delivery of the data to the users, which may be undesirable in view of user experience, e.g., in the case of a live broadcast of multimedia content. Accordingly, it may be difficult to find an appropriate setting for the offset. Moreover, such appropriate setting of the offset may even vary between different service areas within the cellular network, e.g., due to different delays associated with the connection between the BM-SC and the eNBs of the various service areas or due to different capabilities of the eNBs of the various service areas.
Accordingly, there is a need for techniques which allow for efficiently managing the transmission of multicast data in a cellular network.